1. Field of the Invention
The present invention relates to a screen printing paste. More particularly, the present invention relates to a paste for forming a pattern in order to make it possible to form a pattern such as a via hole or a line of electronic parts with a high precision by a screen printing method, to a screen printing method using the paste, and to a baked thick film body therefrom.
2. Description of the Related Art
In recent years, thick-film technologies (thick-film forming technologies) are widely applied to forming conductor circuits, protective films, insulating layers, etc. of electronic parts, wherein at least one type of inorganic powder as a solid component is dispersed in an organic vehicle to form a paste (screen printing paste), and a pattern is formed using the paste by screen printing, followed by baking to form a circuit or the like.
As progress in electronic parts for miniaturization and higher precision has been made in recent years, development for a technology has been required for a high precision pattern forming method according to this thick-film technology so as to form a fine pattern.
It is noted that precision in printing is a factor which exerts the greatest influence on the ultimate pattern precision when the pattern is formed by the thick-film technology. When a common paste is used for printing a fine pattern, there are some cases in which short-circuiting and line breakage occur on the line sections of the pattern, and via holes, etc. cannot be formed owing to blurring, thin and incomplete printing, sagging, etc. Accordingly, it is difficult to print a fine pattern precisely, and the present technology sometimes fails to meet the progress in miniaturization and higher precision for electronic parts required in recent years.
When a paste with an increased viscosity is used which is obtained by modifying the rate of the solid components in the paste or by employing other methods, the blurring and the sagging can be prevented. However, line breakage occurs more often, and it is difficult to print a fine pattern precisely.
The above-described xe2x80x9cblurringxe2x80x9d refers to the phenomenon where a paste spreads out in the course of the step for filling the paste into screen openings and the step for releasing the screen printing plate (paste transferring step). This xe2x80x9cblurringxe2x80x9d mainly affects the printing precision immediately after printing (that is, just after the squeegee has been run). Accordingly, it is possible to evaluate it by observing a printed matter under microscope immediately after printing.
xe2x80x9cThin and incomplete printingxe2x80x9d refers to the state that the region to which a paste has been transferred and the region to which the paste has not been transferred exist together, as a result of insufficient supply and filling of the paste.
Furthermore, xe2x80x9csaggingxe2x80x9d is the phenomenon that a paste which has been transferred slowly spreads out by the influence of gravity after the screen printing plate has been released. Accordingly, it is possible to evaluate it by observing a printed matter under microscope after printing.
As described above, it is considered important to control the viscosity characteristics of a paste in order to precisely print a fine pattern. Thus, evaluation of the viscosity at a specific shearing speed has been carried out conventionally.
However, the shearing speed applied to a paste varies greatly during the various printing steps such as rolling, filling, paste transferring and plate releasing, and the viscosity of the paste changes greatly as the shearing speed varies. Therefore, it is not possible to fully evaluate the printability by measuring the viscosity at a specific shearing speed (i.e., one shearing speed alone), as has been conducted conventionally, and the relationship between the printability and the viscosity has not been fully elucidated.
Under these circumstances, the viscosity control has been carried out conventionally through a trial-and-error approach based on experience. However, it has been becoming difficult to filly meet the requirement of finer print which has been required in recent years by using only such experience.
One of the objects of the present invention is to solve such problems as described above and to provide a screen printing paste with which a fine pattern can be formed precisely, a screen printing method using the screen printing paste, and a baked thick film body formed by using the screen printing paste.
For achieving the above-described objects, various investigations and experiments have been conducted to find the viscosity characteristics for a paste with which a fine pattern could be formed precisely, and as a result, it was found that it was possible to obtain a paste for forming a fine pattern by controlling the viscosities of the paste at least at two different shear rates. The present invention has been accordingly accomplished by further conducting investigations and experiments.
Accordingly, a screen printing paste according to the present invention is characterized in that it has a viscosity of not less than about 40 Paxc2x7sec at a shear rate of 4 secxe2x88x921 and a viscosity of not more than about 210 Paxc2x7sec at a shear rate of 10 secxe2x88x921.
Since the screen printing paste according to the present invention has a viscosity of not less than about 40 Paxc2x7sec at a shear rate of 4 secxe2x88x921, it is possible to secure reliable print without generating blurring of the paste or permitting the paste to reach the rear of the screen printing plate during the step of paste filling/transferring in the course of screen printing, thus without causing short-circuiting of fine lines.
Furthermore, since the screen printing paste according to the present invention has a viscosity of not more than about 210 Paxc2x7sec at a shear rate of 10 secxe2x88x921, good flowability is secured for the paste during the paste transferring, with the result that good printing is secured without causing line breakage even when a fine line is formed by screen printing. A denser pattern is made possible accordingly.
It is noted that the screen printing paste according to the present invention is particularly favorably used for forming a printed pattern containing a line pattern with a line width of not more than about 100 xcexcm. It is also possible to precisely form a printed pattern containing a line pattern with a line width of not more than about 75 xcexcm, or even with a line width of not more than about 50 xcexcm, by also controlling other conditions.
Furthermore, the solid components which are contained in the screen printing paste according to the present invention preferably have an average particle diameter (D50) of less than about 10 xcexcm.
When the average particle diameter (D50) of the contained solid components is less than about 10 xcexcm, it is possible to improve the characteristics for filling the paste into the screen openings, thus preventing line breakage which is related to the size of solid components, and making it possible to precisely print a fine pattern.
Furthermore, the screen printing paste according to the present invention is preferably characterized in that the flowability amount of the paste is not less than about 0 rad and not more than about 0.011 rad when a stress of 10 Pa is applied to the paste for two minutes.
By setting the flowability amount of the paste to be not less than about 0 rad and not more than about 0.011 rad when a stress of 10 Pa is applied to the paste for two minutes, it is possible to restrict blurring due to sagging of the paste and to print a pattern precisely according to the plan. Furthermore, it is possible to prevent lines from short-circuiting due to sagging when the paste has such viscosity characteristics.
It is noted that the above-described flowability amount is a value determined by using a RHEOMETER CSL100 manufactured by Carri-Med, Inc. and a 2 cm longxc3x971xc2x0 cone, and is defined as the rotational angle of the cone when a stress of 10 Pa is applied to the paste using the apparatus for two minutes.
Furthermore, a screen printing method according to the present invention is characterized in that a printed pattern having a specific shape is formed by subjecting the screen printing paste to printing on a subject to be printed using a screen printing plate.
By performing screen printing using a screen printing paste according to the present invention, it is possible to form a fine printed pattern with an excellent shape precision.
Furthermore, in the screen printing method according to the present invention, the above-described printed pattern preferably contains a line pattern with a line width of not more than about 100 xcexcm.
By performing screen printing using a screen printing paste according to the present invention, a printed pattern containing a fine line pattern with a line width of not more than about 100 xcexcm can be formed precisely and without fail, which has not been necessarily easy when a conventional screen printing paste is used.
Furthermore, a baked thick film body according to the present invention is characterized in that it is a baked thick film body having a line made of a thick film with a line width of not more than about 100 xcexcm, and the body is formed by baking a printed pattern obtained by subjecting a screen printing paste according to the present invention to screen printing.
The baked thick film body according to the present invention is formed by baking a printed pattern formed by using a screen printing paste according to the present invention. Accordingly, it is possible to form it with a high level of shape precision when it has a fine line with a line width of not more than about 100 xcexcm.